In U.S. Pat. No. 3,864,456, assigned to the assignee of this application, there is described a process for the production of chlorine dioxide and chlorine wherein sodium chlorate is reduced with added chloride ions in an aqueous acid reaction medium containing sulphuric acid at a low total acid normality in the range of about 2 to about 4.8 normal. The reaction medium is maintained at its boiling point under a subatmospheric pressure, so that chlorine dioxide and chlorine are removed from the reaction medium in gaseous admixture with steam. By-product anhydrous neutral sodium sulphate is deposited from reaction medium once the reaction medium becomes saturated therewith after start up. The gaseous mixture of chlorine dioxide, chlorine and steam removed from the reaction zone is contacted with water, usually after at least partial condensation of the steam, to form a chlorine dioxide solution also containing dissolved quantities of chlorine.
It has previously been suggested in U.S. Pat. No. 3,347,628 to form an aqueous chlorine dioxide solution from a gaseous mixture of chlorine dioxide, chlorine and steam removed from a chlorine dioxide generator to which external steam is added to dilute the gases, by contact of the gaseous mixture with water. In this prior process, the chlorine gas remaining from the absorption is reacted with sulphur dioxide and water to form sulphuric acid and hydrochloric acid, which are fed to the chlorine dioxide generator.
As is set forth in detail in U.S. Pat. No. 4,086,329 assigned to the assignee of this application, the latter concept is not directly utilizable in the process of U.S. Pat. No. 3,864,456, since the chemical efficiency of chlorine dioxide production under boiling reaction medium, subatmospheric pressure and low total acid normality conditions is less than 100% in the latter process. As described in U.S. Pat. No. 4,086,329, a critical adjustment of the hydrogen and chloride ion concentration of the acid feed is required to take this inefficiency into account, otherwise continuous operation is impractical.
It has now been found that the presence of dissolved unreacted sulphur dioxide in the hydrochloric acid and sulphuric acid mixture resulting from the reaction of chlorine, sulphur dioxide and water and forwarded to the reaction medium is detrimental to the chlorine dioxide-generating process, even in very small concentrations. Often cell liquor, i.e. the sodium chlorate solution resulting from diaphragmless electrolysis of sodium chloride solution, is used as the source of sodium chlorate feed to the reaction medium. Such cell liquor usually contains dissolved quantities of sodium dichromate, as a result of the beneficial utilization of such chemical in the electrolysis reaction. The presence of dissolved quantities of sulphur dioxide in the mixed acid stream has been found to reduce the dichromate ions to trivalent chromium, which in turn causes the anhydrous sodium sulphate precipitate to form as very fine crystals which are very difficult to filter or otherwise separate from the reaction medium. Further, in the absence of dichromate ions from the reaction medium, the presence of dissolved quantities of sulphur dioxide in the mixed acid stream has been found to decrease the efficiency of chlorine dioxide production.
A procedure in which the by-product chlorine from the chlorine dioxide absorption is reacted with sulphur dioxide and water to form a mixture of sulphuric acid and hydrochloric acid for reuse in the chlorine dioxide generator nevertheless is a commercially-attractive one. Since the hydrochloric acid is used to provide at least part of the chloride ion requirement and part of the acid requirement for the chlorine dioxide-producing process, the overall amount of sodium sulphate produced per mole of chlorine dioxide produced is decreased when compared to a process wherein sodium chloride provides all the chloride ion requirement and sulphuric acid provides all the acid requirement, as is apparent from consideration of the following equations: ##EQU1## wherein x is the molar proportion of HCl which is used and is a decimal value which is less than or equal to 1.00. Equation (1) represents the reaction which produces chlorine dioxide and the extent to which the reaction of equation (1) predominates over equation (2) is the efficiency of chlorine dioxide production.
It will be seen that the proportion of sodium sulphate which is produced declines as the proportion of hydrochloric acid used in place of sodium chloride and sulphuric acid increases. The requirements of pulp mills for sodium sulphate have declined while requirements for chlorine dioxide have increased. The ability to produce less sodium sulphate through the use of hydrochloric acid, therefore, is beneficial.
Further, since chlorine gas from the absorption is reacted to form reutilizable chemicals, the necessity for separate absorption of chlorine, usually in sodium hydroxide solution to form hypochlorite, is substantially decreased. With the increasing tendency to substitute chlorine dioxide for a substantial proportion of the chlorine which has formerly been used to effect bleaching in the first stage of a multistage bleaching operation, the requirements for chlorine have decreased while those for chlorine dioxide have increased.